1. Field of the Invention
The present invention relates to communications systems for performing communications among a plurality of communications satellites as well as communications among these communications satellites and a plurality of terrestrial communications equipment.
2. Description of the Related Art
It has been known that at least three communications satellites are required in order to cover the entire surface of the earth by satellite communications. A system by which radio communications are established among a plurality of terrestrial communications equipment via either one of the three geostationary communications satellities is known widely as a conventional satellite communications system. In the conventional satellite communications system of this kind, it is necessary that each one of these satellites has to be placed on a geostationary orbit. Moreover, because of the requirement to cover the surface of the earth with a small number of satellites, it is necessary that these satellites be placed on orbits of relatively high altitude of about 40 thousand kilo-meters above the surface of the earth. Because of this, it takes time for radio waves to make a round trip between a terrestrial equipment and a satellite, which increases the communication time. Moreover, there is a problem in that communication signals are apt to be interfered by noise making it difficult to obtain communications of high quality. Further, there is another problem that the transmission power of the terrestrial communications equipment has to be large accordingly.
In order to resolve these problems in the conventional satellite communications, a new satellite communications system, such as IRIDIUM Program, has been proposed in recent years. This is a system in which a large number of satellites are made to revolve around the earth at altitudes lower than the altitudes of the conventional communications satellites, and establish communications between the satellite and the terrestrial equipment. Since the altitudes of the satellites are reduced, it is possible to solve the problem of communication delay and the problem of noises mentioned above. According to this system, however, the altitude of the satellite is to be set low so that it is not possible to make the satellite geostationary. In addition, since it is not possible for a single satellite to cover a wide area, it is required to let a larger number of satellites than in the conventional system revolve around the earth.
The system according to the IRIDIUM Program has a significant feature in that it can resolve the problem related to the quality of communications existing in the conventional systems. However, the utility of the system is limited strictly to the communications from one point to another point on the earth. In spite of the large number of satellites involved, this system is not capable of handling at one's convenience the case of transmitting different data to different receiving destinations from one terrestrial station. In addition, communications among a plurality of ground points of three or more by providing them with a switching function are not feasible by this system, and in that sense the system cannot add a function which supersedes the conventional satellite communication functions.